This invention relates to a process for producing ammonium metatungstate (AMT), and more particularly, relates to producing AMT directly from ammonium tungstate (AT) solution by digestion in the presence of silica.
At present both ammonium paratungstate (APT) and AMT are being used as tungsten sources in the catalyst industry. Since APT has a limited solubility in water (about 2 to 3 percent at room temperature), the user converts APT to a more soluble peroxytungstate form by means of reaction with hydrogen peroxide in order to prepare the catalyst. While AMT is highly water soluble, and thus need not be converted, it is in general more expensive than APT, due to the complexity and/or low yields of processes for producing it.
One of the techniques being used to produce crystalline AMT at the present time involves production from APT by a process described in U.S. Pat. No. 3,175,881. Although this process overcomes some of the disadvantages of the prior art, in practice it is somewhat difficult to control for maximum conversion. As a consequence, on crystallization of AMT from solution, the residual APT must be removed in order to produce completely water soluble AMT products. Another disadvantage of this process is its intermittent batch nature, requiring baking, digesting, filtering and concentrating of solutions before crystallization. On a production scale, such step-wise operations tend to be time consuming and costly.
As an improvement over the above process, a method for producing AMT directly from AT solution without an appreciable formation of APT is described in U.S. Pat. No. 3,591,331. In this process, AT solution is contacted with a liquid extraction system consisting of Di-2-Ethyhexyl phosphoric acid (D-2-EHPA) -- tri-n-butyl phosphate (TBP) -- kerosene which extracts ammonium ion and rapidly lowers the pH to the AMT-forming region. The solution is then digested to allow formation of AMT.
While this method substantially avoids the formation of APT and is ammenable to continuous processing, it has not found widespread usage due to a tendency toward instability of the organic system, and resultant formation of insoluble phospho-tungstates in amounts up to 10 percent or more of the total solid product, depending upon the conditions of formation.
Two additional methods for producing AMT directly from AT solution without the appreciable formation of APT involve contacting AT solution with ion exchange resins, and are the subjects of U.S. Pat. Nos. 3,857,928 and 3,857,929, assigned to the present assignee. While one of these processes is ammenable to continuous processing, U.S. Pat. No. 3,857,928, both processes require periodic resin regeneration.